Dual plug heads?

[KevH]

I'm looking at buying a 1985 FXRS that has dual-plug heads that the owner says were done by Dave Mackie. I have seen dual-plug heads on Shovels, but never on an Evo. What are the advantages/disadvantages of this setup? I'm assuming the pistons and chamber shape would have to complement it.

[kd]

I'm looking at buying a 1985 FXRS that has dual-plug heads that the owner says were done by Dave Mackie. I have seen dual-plug heads on Shovels, but never on an Evo. What are the advantages/disadvantages of this setup? I'm assuming the pistons and chamber shape would have to complement it.

The combustion chamber and piston domes on the evo are much different (and more efficient) than the shovel. The shovelhead hemi style combustion didn't have as efficient swirl as the evo with quench (squish) zones to improve to burn.  Dual plugs improved that in the shovel design.  The evo swirl is improved by the bathtub shaped quench zone on the edge of the chamber.

[Ohio HD]

You don't see many Evo or Twin Cam motors with dual plugs. I guess there's benefit if you're running extremely high compression, or racing which generally means extremely high compression. The main purpose for dual plugs is to insure a complete (or better) ignition of the combustion gasses. Milwaukee 8 heads are dual plug. But they're probably there to ignite the lean mixture of stock motors.

FYI, generally you want to retard the ignition timing by 5° to 10° from stock on an Evo with dual plugs. Higher compression plays a part in that also.

[kd]

You don't see many Evo or Twin Cam motors with dual plugs. I guess there's benefit if you're running extremely high compression, or racing which generally means extremely high compression. The main purpose for dual plugs is to insure a complete (or better) ignition of the combustion gasses. Milwaukee 8 heads are dual plug. But they're probably there to ignite the lean mixture of stock motors.

FYI, generally you want to retard the ignition timing by 5° to 10° from stock on an Evo with dual plugs. Higher compression plays a part in that also.

A good OEM example is the higher compression M8 4 valve engine.  It lives and tunes quite well because of dual plugs.

There is also times dual plugs can assist in low compression cases where the bigger bang isn't produced upon ignition. A low compression turbo engine producing low early rpm compression that results in high top end compression due to the turbo going to work, gets a helping hand in the burn on both ends of the rpm range.  (Suck, Squeeze, Bang, Blow)

[Ohio HD]

A good OEM example is the higher compression M8 4 valve engine.  It lives and tunes quite well because of dual plugs.

That and they get intake flow of 370cfm and up from some porters. 

[JSD]

My mate had branch evo heads on 4 5/8" x 3 13/16 " bore the heads had been welded an bath tubed like TC s are now . We twin plugged it. A power house in the day.

[JW113]

As above, the main advantage for an EVO and later is to allow for less timing (thus less change for detonation) on high compression engines running on low octane (pump) gas.

For a few years, some Sportster modles (XL1200S-Sport?) had factory dual plug heads. These may be Buell heads, I don't really remember.

I have dual plugs on my Ironhead, which is 10.25:1, and timing set back 10 degrees. For a street EVO, I'm sure dual plugs can't hurt, but not sure how much it helps either.

-JW

[jsachs1]

The only time I've used them on a Harley, was when I was building engines for forced induction, re: Aerocharger Turbo Systems.
John

[jsachs1]

You cannot view this attachment.Dual Plugs.
! of quite a few.
John